The present invention relates to a device for monitoring multiple DC electrical currents using sensors and, more particularly, to a device for monitoring performance of multiple solar arrays with the sensors. This is referred to as Multi-Circuit Direct Current Monitor (MCDCM) for purposes of this application.
Many commercial and institutional building owners are installing solar arrays to provide electrical energy that can be used at the facility and, in some cases, sold back to the utility providing power to the facility. There are a number of elements that are monitored in these systems, both to measure the amount of power produced and consumed in the facility and to ensure that the solar panels and arrays are functioning at peak efficiency. A typical large scale (>100 kilowatts) installation has a number of components that combine to produce usable power at the facility level. The primary component of the installation is the solar panel which is a device composed of multiple individual photovoltaic (PV) cells mounted in a weather-proof enclosure for installation on a roof top or other suitable location. Each of these solar panels produces a direct current (DC) power output from the radiation of the sunlight striking the panel. Typical power outputs for commercial solar panels are in the range of 50 to >200 watts, with voltages from 17 to 35 volts DC. For increased efficiency, multiple solar panels are electrically connected into “strings” of up to 12 individual panels to provide a single DC output for each string that sums the power from each individual panel. The output from each of these strings is then wired into a combiner, which sums the power of multiple strings into a single DC output. The DC power from the combiner (either singly or in series with other combiner boxes) is then sent to an inverter, which converts the DC output of the total solar array into 60 Hz alternating current (AC) which can then be used by the facility for its power needs. In some cases, the facility owner may contract with its existing utility to sell the power back to the utility if there is solar generating capacity that exceeds the needs of the facility itself.
For most commercial installations, the only required monitoring is the metering device that measures the AC power generated from the inverter of the solar array and the power consumed by the facility to meet its needs. This may require one or more meters to measure the bi-directional flow of AC power and is generally referred to as a “net meter”, a term which simply refers to the ability of the meter(s) to measure the net amount of power consumed by the facility less the power produced by the solar array (or other local generating sources such as generators). The net amount will be positive (i.e. the facility owes a dollar amount to the utility) or negative (i.e. the facility has produced power in excess of its needs and is owed money from the utility for this production). These meters are generally specified by the utility in conjunction with the installer and will provide outputs (generally Modbus or pulse) that can be monitored by the utility or a third party data acquisition system. These meters are used for billing purposes, but they provide little if any useful information about the operation of the strings or arrays of the solar system other than to provide a summary of the AC power output from the inverter.
The purpose of the solar array is, of course, to provide power output whenever conditions are conducive to the production of power from the available sunlight. The theoretical maximum solar energy available is 1000 W/m2 (based on the amount of solar radiation at the equator at noon on an equinox day) and the efficiency of a PV cell is a measure of the percentage of the maximum power potential and the actual output of the cell. For example, a PV cell with a 12% efficiency would produce approximately 120 W/m2 (1000 W/m2×12%) at noon at the equator on an equinox day. Most commercial PV panels in use today have rated efficiencies of between 10% and 20%. The actual output of any given panel is affected by a number of factors, including the geographic location of the array, the angle of incidence, and the number of days of sunshine. The output of the panel may also be impacted on a short term basis by cloud cover, dirt, obstructions, or failure of any of the electrical components or connections. The net meter previously described can only provide a general indication of the performance of the array, but cannot provide additional information regarding any of the components in the overall solar system.
There are several external factors which may be measured in order to determine if the solar array and its individual components are functioning properly and providing suitable power output. Environmental indicators that may be monitored include (but are not limited to) the following: solar radiance (in W/m2), temperature (in ° F. or ° C.), humidity (in % RH), wind speed (in miles/hour or km/hour), and wind direction. These can either be measured using individual sensors connected to an input/output module or the sensors may be incorporated into a weather station package that provides a serial output (e.g. Modbus RTU) that can be read by a computer or a data acquisition server. In the case of the weather station, the single serial output provides data for each of the connected devices/components. Monitoring these environmental factors allows the owner/installer to determine what impact (if any) changes in weather condition had on the expected performance of the solar array. For example, measuring the solar radiation (using a pyranometer) provides a basis for evaluating the impact of smog or haze on the output of the solar arrays and the other environmental factors can be considered in a similar manner to compare actual performance versus expected performance of the array.
The external environmental factors previously described can provide insight into the performance of the solar array, but they do not provide a means for locating and identifying other issues which may arise within the solar power system. Monitoring the inverter(s) on the solar system can provide information regarding the operation of the inverter itself, in particular the efficiency of the inverter in converting the incoming DC power into useful AC power. Tracking the input DC power and the output AC power (in conjunction with the weather monitoring system described previously) can help the owner/installer to identify problems that arise such as electrical failures or obstructions on the solar array as a whole, but does not provide information regarding the source of the inefficiency unless the inverter itself is at fault. For example, dirt or debris can accumulate on the surface of the solar panel and this will greatly reduce the amount of power produced by the panel as the sunlight is prevented from reaching the solar cell. This information can only be gained by monitoring DC power output at the panel level or the string level and using this data to determine if one or more panels are not operating at expected efficiency. Monitoring of DC power at the string level (typically 12 panels per string) provides sufficient accuracy to permit identification of problems at the individual panel or string that affect power output. This can be accomplished by using individual Hall Effect sensors or shunts to measure the DC current from each string, but this approach requires significant space for installation and also requires a number of additional devices and installation labor to bring each of the signals into an analog input device for communication to the data acquisition system as well as significant configuration labor to provide appropriate scaling factors.
What is desired, therefore, is a sensing device which provides DC current sensing for multiple power feeds into a single device which continuously measures the DC current from each string and provides an output for all desired parameters. This device may be suitable for both new installations and for retrofit into existing arrays after installation has taken place. This device may be accurate enough to provide indication of the failure of any PV cell in the string. The device may also provide for a comparative analysis of the monitored circuits to provide indication of failed or failing panels based on a threshold of variance for one circuit from the value of other circuits, allowing for accurate reporting of failures across a wide spectrum of solar performance due to external factors (e.g. cloudy day).
The foregoing and other objectives, features and advantages of the invention will be more completely understood upon consideration of the following detailed descriptions of the invention, taken in conjunction with the accompanying drawings.